Recently, in order to improve information processing capability in electronic devices, frequencies of electrical signals transmitted through circuits on wiring boards have been increased. Consequently, even if the frequencies of electrical signals are increased, it is desired to maintain electrical reliability of wiring (circuit) boards and to prevent decreases in the transmission speed of electrical signals and loss of electrical signals in the circuits.
Meanwhile, the circuit boards are usually provided with protective films for protecting the wiring boards and circuits, and insulating layers, such as interlayer insulating films for ensuring insulation between the individual layers in multilayer wiring boards. Since the protective films and the insulating layers, such as interlayer insulating films, are disposed on the wiring boards, they are required to have adhesiveness so that they are bonded to the wiring boards, in addition to insulating properties. In particular, when multilayer wiring boards are manufactured by laminating flexible printed circuit boards (FPCs), build-up circuit boards, or the like, the individual boards are bonded and fixed to each other by the interlayer insulating films. Therefore, the interlayer insulating films are required to have excellent adhesion to the boards or the like. Accordingly, the protective films and the insulating layers, such as interlayer insulating films, are formed using adhesive materials having adhesiveness.
Consequently, in order to improve information processing capability in electronic devices by increasing the frequencies of electrical signals, even if insulating layers are formed using adhesive materials, it is desirable to obtain high reliability of wiring boards in the GHz (gigahertz) range and to avoid adverse effects on the transmission of electrical signals.
In the past, as the adhesive material for wiring boards, for example, an epoxy adhesive material or a thermoplastic polyimide adhesive material has been used. The epoxy adhesive material has excellent processability, such as capability of bonding adherends under low-temperature and low-pressure conditions, and also has excellent adhesiveness to adherends. The thermoplastic polyimide adhesive material has excellent heat resistance, such as low thermal expansion and high thermal decomposition temperature.
Furthermore, Japanese Unexamined Patent Application Publication No. 8-27430 (Publication Date: Jan. 30, 1996 (Heisei 8)) describes that use of a film adhesive prepared by mixing a polyimide resin having a glass transition temperature in a predetermined range, an epoxy compound, and a compound having an active hydrogen group reactive with the epoxy compound enables low-temperature, short-time bonding between adherends and achieves heat resistance reliability at high temperatures.
However, an epoxy resin obtained by curing the epoxy adhesive material has a dielectric constant of 4 or more and a dielectric loss tangent of 0.02 or more in the GHz range, resulting in a problem that satisfactory dielectric characteristics cannot be obtained.
Furthermore, with respect to an adhesive material using a polyimide resin obtained by curing the thermoplastic polyimide adhesive material, in order to allow adherends to adhere to each other using such a material, the adherends must be bonded to each other under high-temperature and high-pressure conditions, causing a problem in processability.
Furthermore, the film adhesive described in Japanese Unexamined Patent Application Publication No. 8-27430 can be processed at low temperatures for a short period of time and has excellent heat resistance reliability at high temperatures. However, the publication does not describe dielectric characteristics. The epoxy compound contained in the film adhesive described in Japanese Unexamined Patent Application Publication No. 8-27430 decreases the softening temperature of the film adhesive to improve low-temperature processability. However, if a large amount of the epoxy compound is incorporated, the dielectric constant and the dielectric loss tangent are increased, resulting in a degradation in dielectric characteristics.
Therefore, in order to improve information processing capability in electronic devices by increasing the frequencies of electrical signals, it is desired to develop an adhesive material which is excellent in adhesiveness, processability, and heat resistance and which is capable of forming insulating layers having excellent dielectric characteristics, i.e., exhibiting a low dielectric constant and a low dielectric loss tangent, even in the GHz range.
However, so far, with respect to resin compositions containing epoxy compounds and thermoplastic polyimide resins, there has not been provided a thermosetting resin composition which can be suitably used for manufacturing circuit boards, such as flexible printed circuit boards and build-up circuit boards, which is excellent in flowability, adhesiveness, processability, and heat resistance, and which has excellent dielectric characteristics in the GHz range.
Furthermore, recently, in view of environmental concerns, with respect to various materials used for electronic devices, there has been a need to take recycling into consideration and to avoid the use of environmentally unfriendly substances as much as possible. For example, with respect to flame retardants, non-halogen (halogen-free) flame retardants are desired, and with respect to solders, solders that do not contain lead (lead-free solders) are desired.
In particular, with respect to solders, in the past, eutectic solders containing lead have been mainly used as materials for physically and electrically connecting wiring boards and mounted parts. However, in view of environmental concerns, the lead-free solders that do not contain lead have come into common use. The lead free solders have melting points that are about 40° C. higher than those of conventional solders containing lead. Therefore, with respect to materials for wiring boards, there has been a strong demand for further improvement in heat resistance.
As resin materials used for wiring boards, in particular, adhesive materials and insulating materials used for the insulating layers, such as interlayer insulating films, and the protective films, curable resin compositions including polyimide resins and cyanate ester compounds are known as described above. However, such curable resin compositions have problems with respect to dielectric characteristics and processability. In order to overcome the problems in the epoxy adhesive materials and the thermoplastic polyimide adhesive materials, as a different system from that of resin compositions containing epoxy compounds and thermoplastic polyimides, Japanese Unexamined Patent Application Publication No. 2001-200157 (Publication Date: Jul. 24, 2001 (Heisei 13)) discloses a thermosetting resin composition containing a polyimide resin and a cyanate ester compound as an adhesive material (resin material) having excellent dielectric characteristics and processability.
Furthermore, Japanese Unexamined Patent Application Publication No. 8-8501 (Publication Date: Jan. 12, 1996 (Heisei 8)) discloses a low dielectric constant multilayer printed circuit board which contains a cyanate ester compound as a main component and which has flame retardance. Furthermore, Japanese Unexamined Patent Application Publication No. 9-132710 (Publication Date: May 20, 1997 (Heisei 9)) discloses a polyimide resin composition having excellent adhesiveness.
However, these conventional techniques, in particular, in the application of manufacturing wiring boards that meet the requirement of improving information processing capability in electronic devices, have difficulty in improving various physical properties of resin materials in a well-balanced manner. More specifically, with respect to the conventional techniques, in the resin materials used in the above-described application, it is difficult to satisfy the requirements of both flame retardance and other physical properties, such as heat resistance, processability (including solvent solubility), and dielectric characteristics.
For example, in the thermosetting resin composition disclosed in Japanese Unexamined Patent Application Publication No. 2001-200157, a polyimide resin is mixed with a cyanate ester compound, and this thermosetting resin composition is effective in satisfying the requirements of dielectric characteristics, heat resistance, and processability. However, there is no description in the publication of flame retardance, which is an important property of the material constituting circuit boards. Therefore, it is not clear if the thermosetting resin composition has sufficient flame retardance.
The low dielectric constant multilayer printed circuit board disclosed in Japanese Unexamined Patent Application Publication No. 8-8501 is composed of a material prepared by mixing a cyanate ester compound and brominated bisphenol A. Because of the use of the brominated phenol, the multilayer printed circuit board has flame retardance that is sufficient for use in a wiring board. Furthermore, the polyimide resin composition disclosed in Japanese Unexamined Patent Application Publication No. 8-8501 includes a halogen atom or a halogen-containing hydrocarbon group. That is, in spite of the fact that recently flame-retardant materials that do not include halogen compounds have been strongly desired in view of environmental concerns, a halogen compound is used.
Accordingly, so far, there has not been provided a thermosetting resin composition in which it is possible to sufficiently satisfy the requirements of both flame retardance and other physical properties, such as dielectric characteristics, heat resistance, and processability (including solvent solubility), in view of environmental concerns, and in particular, which can be suitably used in manufacturing wiring boards that satisfactorily meet the requirement of improving information processing capability in electronic devices.